The present invention relates to mass spectrometers.
A mass spectrometer analyzes mass-to-charge ratio of particles, such as atoms and molecules, and typically includes an ion source, one or more mass analyzers and one or more detectors. In the ion source, sample particles are ionized. The particles can be ionized with a variety of techniques using electrostatic forces, laser beams, electron beams or other particle beams. The ions are transported to one or more mass analyzers that separate the ions based on their mass-to-charge ratios. The separated ions are detected by one or more detectors that provide data that is used to construct a mass spectrum of the sample.
The ions can be guided, trapped and analyzed by devices such as multipole ion guides or linear or 3D ion traps. For example, multipole rod assemblies, such as quadrupole, hexapole, octapole or greater assemblies, include four, six, eight or more multipole rods, respectively. In the assembly, the multipole rods are arranged to define an internal volume, such as a channel or a ring, in which the ions can be trapped or guided by applying radio frequency (“RF”) voltages on the multipole rods. Depending on the applied voltage, the rod assembly can selectively trap, guide or eject ions that have particular mass-to-charge ratios.
For example, a linear ion trap can be used as a stand-alone mass analyzer by applying voltages that eject particles corresponding to different mass-to-charge ratios, and detecting the ejected particles. Alternatively, linear traps can be used in tandem mass spectrometry to isolate or activate particular ions that will be analyzed by another mass analyzer, such as a Fourier transform ion cyclotron resonance (“FTICR”) mass analyzer. At isolation, all particles are ejected from the trap except ions within a narrow range of mass-to-charge ratios, called the isolation mass range, that corresponds to masses of target molecules. At activation, the isolated ions, called parent ions or precursor ions, are excited and eventually fragmented into their basic building blocks. Ionized fragments are called daughter ions or product ions. The activation can be performed by applying an AC voltage to multipole rods with an activation frequency corresponding to a resonant frequency of the precursor ions. The mass spectrum of the product ions can be used to determine structural components of the precursor ions.
In a multipole ion trap or ion guide, ions are manipulated by electric fields generated by the voltages applied to the multipole rods or other electrodes of the ion trap or ion guide. In addition to the electric fields generated by the applied voltages, the ions are also subject to electric fields that are generated in the ion trap or ion guide by the ions themselves. The self-generated electric fields have a characteristic strength that increases with the size of the ion population in the ion trap or ion guide. Conventionally, the ion trap or ion guide is operated with ion populations for which the self-generated electric fields are substantially smaller than the applied electric fields. Thus, the number of ions in the ion population is traditionally limited to avoid self-generated fields that may affect one or more particular operations. Such limits are known as space charge limits.